Quality music for a small choir

I am a new choral director for a very small rural school. My choir has 15 members in it. I need some help finding quality literature for them. We have an accompaist who is fairly good but I haven't found much for quality music for the choir. I would like to do Lauridson's O Magnum Mysterium that is too much for this size of a choir. Can anyone please recommend pieces that would be good for my choir?

My choir is a mixed choir. I generally use SAB music for them. It is a high school choir. They range from 9-12th grade. They aren't very experianced for the most part, there are two who are though. I would like music that is accessible to them but would also challenge the two who are very experianced.